Recently, it has been proposed to add dispatch capabilities to the cdma2000 system. Unlike the interconnect services provided by today's cellular systems, dispatch services have been traditionally provided by two-way radio systems. Such services allow a user to communicate in ways that are difficult or costly using today's cellular systems. The dispatch group call service, for example, enables a user to communicate with one person or a group of people simultaneously and instantaneously, usually just by depressing a push-to-talk (PTT) button. Using a traditional cellular system, such a call could not occur instantaneously because telephone numbers would need to be dialed for a three-way call or arrangements would need to be made to set up a conference call.
Likewise, the dispatch individual call service enables a user to communicate with another user quickly and spontaneously. This feature is ideal for two people who are working together but are unable to speak with one another directly such as two people working in concert but in different locations in a building. Whereas a wireless telephone call is more appropriate for a conversation, communication between two people as they work is better facilitated by the dispatch individual service.
Dispatch private and group calls are low cost alternatives to interconnect calls. Dispatch voice is being targeted for transmission using radio link protocol (RLP) to take advantage of increased capacity due to increased frame error rate (FER) which is tolerable when one uses retransmissions. Currently, RLP running at a sending device, such as a mobile station, attaches a sequence number to a frame before it is transmitted to a receiving device, such as a base station controller (BSC). The sequence number is used by the RLP running at the receiving device to determine when a frame has been erased so that it can send an acknowledgement of the erasure (or a Negative Acknowledgement (NAK) of the frame erased) to the sending device. Upon receiving the acknowledgment or NAK, the sending device generally retransmits the frame. However, if the sender has discarded the retransmission due to a memory limitation, no retransmissions occur. Other exceptions could also exist.
U.S. Pat. No. 6,169,732 issued on Jan. 2, 2001 to Hetherington et al. and assigned to Motorola, Inc., discloses a method and apparatus in which a communication link between a first and second mobile communication resources is established via a series of land-based communication resources. The first mobile communication resource transmits, in sequence, sequentially numbered packets of data via the first wireless connection to a first land-based communication resource for subsequent transmission to the second mobile communication resource via the communication link. A detecting communication resource detects reception of out of sequence packets of data received at a receiving portion of its transceiver section and requests retransmission of the missing packets. For example, if the first mobile communication resource transmits frames 1, 2, 3 and 4, then upon detecting that frame 3 is missing, the first land-based communication resource requests retransmission of the missing frame 3. Thus, in current RLP, detection of an erased frame is delayed by at least one frame. This approach causes delay in requesting retransmission of erased frames.
Thus, there is a need for a modified RLP capable of detecting frame erasures closer in time to when they occur to decrease the delay in requesting retransmission of erased frames.